Electronic matrix arrays find considerable application in devices such as imagers (e.g. X-ray imagers) and LCDs. Such devices typically include x and y (or row and column) address lines which are horizontally and vertically spaced apart and cross at an angle to one another thereby forming a plurality of crossover points. Associated with each crossover point is an element (i.e. pixel) to be selectively addressed. The element may be a pixel of an imager array, or alternatively a pixel of an LCD.
Typically, a switching or isolation device such as a thin film transistor (TFT) is associated with each element or pixel in order to permit individual pixels in the imager or LCD to be selectively addressed (i.e. driven in LCD applications and read in imager applications).
Structurally, TFTs typically include source, drain, and gate electrodes, with a thin film of semiconductor material (e.g. amorphous silicon or a-Si) disposed between the source and drain electrodes, and the gate electrode in proximity to the semiconductor but electrically insulated therefrom by a gate insulator. A typical bottom-gate TFT is shown, for example, in U.S. Pat. No. 5,641,974, the disclosure of which is hereby incorporated herein by reference.
Current flow through the TFT between the source and drain electrodes is controlled by the application of voltage to the gate electrode. As shown in FIG. 1, the application of voltage to the gate produces an electric field which accumulates a charged region in the semiconductor layer (e.g. intrinsic a-Si). This charged region forms a current conducting channel in the semiconductor through which current flows between the source and drain electrodes of the TFT. For example, when zero or a negative voltage is applied to the gate, the TFT will be in an off-state and little or no current will flow between the source and drain. However, when a positive voltage (e.g. +10 volts) is applied to the gate, this causes the conducting channel to form and permits current to flow between the source and drain electrodes of the TFT.
FIG. 1 is a symbolic diagram of a typical TFT in an LCD showing a current path from the drain terminal to the source terminal (A.fwdarw.B.fwdarw.C.fwdarw.D) when the n-channel TFT turns on (i.e. V.sub.d &gt;0 volts, V.sub.g &gt;V.sub.th, V.sub.s =0 volts). R.sub.s, R.sub.d, and R.sub.ch represent source series resistance, the drain series resistance, and the channel resistance, respectively. V.sub.d is the drain voltage. V.sub.g is the gate voltage. V.sub.s is the source voltage. V.sub.th is the threshold voltage (e.g. 1 volt for n-channel TFTs) which when exceeded results in the TFT being in the on-state.
Herein, the "source" electrode of a TFT will be referred to as the electrode which is to be electrically in communication with a pixel electrode (e.g. connected to a pixel electrode in an LCD or to a pixel or collector electrode in an imager). Thus, the "drain" electrode of TFTs herein will be referred to as the electrode which electrically communicates with a data or address line and which is spaced from the corresponding source electrode.
Thus, in LCD applications for example, when a voltage (e.g. +10 volts) is applied to the gate and at the same time a video voltage (e.g. +5 volts) is applied via a data address line to the drain of a TFT, the conductive channel is formed in the semiconductor layer and current flows therethrough from the drain to the source powering the pixel electrode in the LCD so as to charge the corresponding liquid crystal (LC) pixel of the display. The pixel is then in an "on-state." In LCD applications, the source typically reaches a voltage similar to that supplied to the drain via the data line in the on-state. The amplitude of voltage applied to the drain via the data line thus determines how much voltage will be applied across the liquid crystal material in a given pixel and thus controls gray scale levels in the display. When voltage is no longer applied to the gate (or drops below V.sub.th), the pixel stops charging but remains on until the next frame. Unfortunately, TFT leakage current can cause current to leak from the source to the drain through the semiconductor layer when the pixel is in an on-state and the TFT is in an off-state, thereby unintentionally and undesirably changing the amount of voltage across the liquid crystal (LC) material in the pixel and adversely affecting gray scales and shading in the display (i.e. undesirable discharging of the pixel). This leakage current is an undesirable characteristic associated with conventional TFTs.
In, for example, X-ray imager applications, the source of a TFT is typically electrically connected to a collector electrode of a given pixel. The collector electrode receives a voltage which is a function of the amount of X-ray radiation received in a corresponding area of the imager. A pixel or collector electrode in an imager may receive from about 1 to 30 volts, depending upon the amount of radiation received by that area of the imager. The drain electrode in imager TFTs is typically connected to a data line having a constant voltage (i.e. virtual ground line defining a reference voltage such as 0 volts, 5 volts, or the like). When a voltage is supplied to the gate of a TFT in an imager in order to cause the TFT to go into an "on-state", current flows from the source to the drain so that the pixel or storage capacitor may be "read" with the voltage read from the collector electrode indicating how much radiation a given area of the imager received in a frame. Again, leakage characteristics of TFTs may allow current to flow from the source to the drain when no voltage is applied to the gate thereby allowing the imager pixel (or storage capacitor) to unintentionally discharge. This is undesirable because the voltage read out when the TFT is turned on will not be directly indicative of the radiation received.
Accordingly, a TFT's leakage current is a critical parameter determining the overall image quality of both LCDs and imagers (e.g. scanners, X-ray imagers, etc.) It is known that high TFT leakage current degrades performance of a display by yielding non-uniform gray levels, crosstalk, shading, flicker, and/or image sticking, and degrades imager performance by yielding dynamic range reduction, shading, and/or non-uniform image. In typical LCD or AMLCD applications, TFTs are biased such that V.sub.ds ranges from 0 to 10 volts in the off-state, and V.sub.gs from -3 to -6 volts in the off-state. In X-ray imagers, TFTs operate at higher V.sub.ds values in the off-state (i.e. collector or pixel electrodes in such imagers can receive up to 30 volts in certain applications). Thus, TFTs in X-ray imagers and other imagers are in need of extremely low leakage current characteristic in order to minimize leakage when TFTs are in the off-state.
Numerous attempts have been made to minimize TFT leakage current. For example, for crystalline-silicon and polycrystalline silicon TFTs, LDD (lightly doped drain) or drain offset structures have been introduced. However, that approach requires additional process steps (i.e. photo, ion implantation, etc.).
Leakage current in amorphous silicon (a-Si) TFTs, such as that disclosed in U.S. Pat. No. 5,641,974, has been understood as follows. Firstly, bulk leakage current may be caused by hole conduction. Holes are provided via thermal generation, field enhanced thermal generation (Pool-Frenkel barrier lowering), and/or tunneling (band-to-band or trap-to-band) in the high electric field region near the source/drain. Secondly, bulk leakage current may be caused by the introduction of density-of-state peak in the upper-half of the energy band gap. Thirdly, surface leakage current may be caused by band-bending at the interface between the top channel layer (e.g. a-Si layer) and the dielectric or passivation layer overlying same. This leakage current is sensitive to density-of-state or fixed charge at the interface, and only applies to bottom-gate TFTs. Finally, gate leakage current can be caused by any leakage via the gate dielectric layer (e.g. silicon nitride layer).
With reference to the immediately preceding paragraph, the bulk leakage current caused by carrier generation due to high electric fields has been found to be the most dominant, and the most difficult to address in terms of manufacturability of imagers and/or LCDs without significantly changing TFT structures or adding process steps.
It is apparent from the above that there exists a need in the art for an improved TFT structure and method of making same, where the TFT, or an array thereof, may be used in, for example, imager or LCD applications. The new structure and method should result in TFTs with reduced leakage current and increased mobility, without adversely affecting TFT manufacturing and/or processing steps.
Conventional TFTs typically have a contact layer between (i) the intrinsic semiconductor layer, and (ii) the source and drain electrodes. For example, see U.S. Pat. Nos. 5,641,974 and 5,532,180. These contact layers may be a-Si doped with an impurity such as phosphorus as disclosed in these two patents. These contact layers have a substantially constant doping density throughout the entire thickness of the contact layers, which results in off-state leakage characteristics in certain applications.
It is a purpose of this invention to fulfill the above-described needs in the art, as well as other needs which will become apparent to the skilled artisan from the following detailed description of certain embodiments of this invention.